2021, so single piece smooth roof? Older roofs had screws into the framing which made this easier. In both cases, FWC will say that they don't have measurements and each camper is custom built, so you have to find these supports yourself. You will want to find the ones that run front to back for mounting rails.
In your case, I would remove screws (not every one, but at least 3) from the inside of the camper, and use a long very thin drill bit and drill through the now vacant scew hole up and through the roof. Do this front, centre and middle, and you have a good guide for the other holes you will need to make.
When I did mine I put a layer of butyl tape under the rail, and applied 3m 4200 sealant to the screws when putting those in.